* 
216 O. W. Huntington—Spectrum of Arsenic. 
purity of the material used, and also that the diffused spectrum 
above referred to cannot come from the material of the tube, 
that no trace of the sodium line was seen. No account was 
taken of the diffused spectrum, as it appeared only when the 
to express that the lines are very faint and too numerous to 
measure. And we wish to call particular attention to the fact 
g 
minimum deviation of the most prominent Fraunhofer lines, 
and verified and somewhat multiplied the data by measuring 
also the angles for characteristic lines of the hydrogen, lithium, 
sodium, thalium and strontium spectra. These we com ined 
with the wave-lengths of the same lines given by Angstrom, 
by ordinates and abscissas in the usual way, and the curve 
usually expressed. 
e instrument is capable of reading to five seconds of are, 
and with the full bank of ten prisms it would give the wave- 
lengths to tenth-meters with perfect accuracy. With the com- 
paratively feeble light of the arsenic spectrum, as We first 
observed it, we did not think it advisable to use the full pow® 
of the instrument. We therefore used five prisms, as state, 
and read to. one minute of are. We always began each seres 
of observations by setting the cross-wire of the micrometer 0” 
* This Journal, xl, November, 1865, 
